As disclosed in JP-A 11-231008, there has been a capacitor life assessment device for detecting a capacitance shortage of an electrolytic capacitor built in a power unit in order to assess the life of the electrolytic capacitor. This conventional capacitor life assessment device is adapted to sample the voltage of a capacitor after the charging thereof and assess the life of the capacitor based on a time constant derived from the sampled voltage.
Further, JP-A8-29465 discloses a capacitor capacitance change detection circuit that determines a capacitance shortage of a capacitor from a period of time until the charging voltage of the capacitor reaches a reference voltage. In this conventional capacitor capacitance change detection circuit, the period of time until the charging voltage of the capacitor reaches the reference voltage is measured by an external comparator (hardware comparator) connected to a CPU. The CPU determines a capacitance shortage of the capacitor by reference to information from the comparator.
In the conventional capacitor life assessment device, however, complicated calculations such as logarithmic calculations are required in order to assess the life of the capacitor. This complicates the processings of the calculations, lowers the speed of the processings, and leads to a setback for cost reduction as well.
Further, in the conventional capacitor capacitance change detection circuit, since the comparator is externally connected to the CPU, the soundness of the comparator itself must be checked independently of that of the CPU, and thus the soundness check of the comparator becomes a troublesome task. This makes it difficult to enhance the reliability of the capacitor capacitance change detection circuit.